1. Technical Field
This invention relates to video display systems and, more particularly, to a method and apparatus for providing concatenated lines and filled polygons in a video display.
2. Description of the Prior Art
Various display systems have been proposed and used heretofore employing interactive computer systems wherein the user manipulates the information stored in or generated by a computer. This information may constitute text, graphics, facsimile, video and the like. In the recent past the advent of very large scale integrated circuitry has made possible the wide use of low cost microcomputer systems. With such a system it is now cost effective to maintain central data bases from which information can be accessed by the general public.
Two information systems using central databases, viewdata and teletext, are currently being considered for wide use in the telecommunications industry. In the viewdata system, the consumer is provided a two-way interactive service capable of displaying pages of text and pictorial materials on a video display. In the teletext system, the consumer is provided with a one-way broadcast information service for displaying pages of text and graphic material on a video display.
In both the viewdata and teletext systems it is necessary to include an electronics module at the consumer end that provides the display control information. In the present invention this module provides the necessary control to the consumer display terminal (such as an ordinary television set) for assembling and displaying an image made up of concatenated lines and filled polygons.
One scheme for providing the control for this type of display is implemented by using a chain link encoding technique wherein the chain link code gives the starting coordinates of a point on an image and then sequences through the coordinates of each subsequent point in defining the lines or border of a polygon. In a rectilinear display array there are eight possible directions that a display point can move to an adjoining point in defining a line or border of a polygon. And three bits of data are generally used to indicate the location of the coordinates for the next point. In an effort to minimize the number of bits in defining the direction of the display point, some arrangements are designed to use two bits rather than three. These arrangements still require the third bit or additional data, however, to provide information as to whether the line defined by the display point as it moves from one set of coordinates to another is written or not written onto the screen. While such arrangements have been found satisfactory where there is both ample bandwidth for transmitting and memory available for storing the data, in those systems where cost is a consideration it is desirable to minimize the total amount of data requirements required to describe an image.